Maternal-fetal transmission is felt to be responsible for a larger number of chronic carriers of HBsAg. The mechanism by which the virus is transmitted from mother to infant is, however, unclear. The three main possibilities are: (1) transplacental transmission prior to birth, (2) swallowing of maternal blood at the time of delivery, and (3) post-partum transmission via breast feeding or maternal handling. We will attempt to differentiate each of these possibilities by delivering infants of HBsAg-positive chimpanzees by Caesarian section and by having each infant reared separate from its HBsAg-positive mother.